The invention relates to a magnetic drive metering pump in which a movable thrust member fixed to a connecting rod is axially movable in a longitudinal axis in a magnet shroud anchored in a pump housing. A compression spring or recuperating spring, with uncontrolled magnets, keeps the thrust member from an inner face of the magnet shroud so that an airgap is formed between the two faces. The thrust member with the connecting rod, on electrically driving (actuating) the magnetizing coil, is drawn into the magnet shroud against the force of a recuperating spring, reducing the airgap, into a bore in the magnet shroud and after deactivating the magnet the thrust member is returned to the starting position by the recuperating spring so that the thrust member and an elastic displacement member actuated thereby carries out an oscillating motion on continued activation and deactivation of the magnetizing coil, which diaphragm cooperates alternately with an outlet and an inlet valve to produce a pump stroke (pressure stroke) and a priming stroke in a metering head arranged in the longitudinal axis,
Magnetic drive metering pumps similar to the above are generally known and are matched to requirements by add-ons. They operate volumetrically, wherein metering is carried out by transporting a closed volume. The metered volume per stroke thus corresponds to the difference in volume on movement of the diaphragm.
In such magnetic drive metering pumps, as previously discussed, a movable thrust member is mounted in a stationary magnet shroud so that when the magnetic coil is driven, it is drawn into the magnet shroud, the airgap shrinks and after switching off the electric drive, the thrust member is impelled back into its starting position by a recuperating spring. A connecting rod is fixedly associated with the thrust member and transfers the motion and force to the metering diaphragm.
In the simplest case, the stroke magnet is switched on for a particular period to execute a metering stroke. Other embodiments supply the magnetizing coil with a controlled current in accordance with a predetermined time profile, wherein the magnetic force and thus the metering performance is more reproducible and independent of electrical parameters such as the actual power of the mains.
The stroke frequency is given by the repetition frequency of the electrical drive pulse. The stroke length can, for example, be altered by means of a mechanically adjustable spindle which sets the start point of the stroke motion; the end point is given when the magnet has moved in completely. In one possible embodiment, a stroke adjustment pin is screwed into a thread in the pump housing and has a calibrated knob accessible from the outside, the back of which is fixed to the magnet shroud or its position is fixed with respect to the magnet shroud.
The motion of the diaphragm occurs by a combination of the effective forces. After switching on, the magnetic current and therefore the force produced initially rises, slowed by self-induction; when the force on the connecting rod generated by the diaphragm and the recuperating spring is overcome, the thrust member begins to move. The airgap shrinks and the corresponding magnetic force rises further. The thrust member accelerates quickly and impinges against the shroud, curbed only by an O-ring which is generally present. The entire movement is executed in a few milliseconds, resulting in very high instantaneous speeds for the metering medium and high pressure peaks, up to twice the operating pressure and beyond.
The diaphragm is not rigid, but deforms elastically by a particular amount in the flexing region when the pressure of the metering medium operates thereon. The amount of the deformation is lost to the effective stroke motion and the result is that with increasing operational pressure, the metered amount reduces. This drop-off characteristic is much more prominent in normal use than allowed by the metering accuracy. Thus, magnetic drive metering pumps normally cannot be used over a wide range of operating pressures with the desired accuracy; moreover, the errors which arise by calibration are exacerbated as they are included in further calculations. However, said calibration measurement must be carried out in use under actual operating conditions and particularly when using aggressive chemicals, is a step which is extremely difficult.
Current magnetic drives have just a few simple parts and thus are easy to manufacture, but are relatively limited in application and suffer from disadvantages as regards the hydraulic properties of the metering process compared with a motor-driven pump. The motor drive, which is transferred, for example by means of a gear or cam, has more applications and for many processes it has better metering properties, but is much more expensive to produce.